If PMUs are only placed on some of the nodes of the system, there will be missing features in
\(X\) .
We overcome this problem by
placing PMUs such that all the nodes either have PMUs on them or are adjacent to at least one node that has a PMU on it.
Such optimal locations can be determined from prior literature on optimal PMU placement for LSE (e.g., [1]}).
Note that loss of PMU data can result in the feature matrix becoming unobservable.
Contrary to creating a separate factor-graph to overcome this issue as done in [2]}, we suggest detecting and replacing bad/missing PMU data before it enters the GNN as already done for regular DNNs in [3]}.
